The use of blow-molding as a method for manufacturing various sorts of articles is generally well known. Typically, this process involves the use of a mold consisting of two separate halves or portions having cavities of particularly desired shapes and sizes. Usually, one extrudes a large-diameter, sealed tube of molten material (commonly referred to as a “parison”), places the tube between the mold halves, and closes the mold around the tube. Fluid pressure is then introduced into the tube, forcing the molten tube against the walls of the cavities, conforming the tube to the shape thereof. The pressure is maintained until the molten material cools and solidifies. The pressure is then released, the mold halves are pulled apart, and the hardened article is ejected therefrom.
Before the blow molded part is removed from the mold, it is typically separated from excess parison material called flash. This step can be completed in-mold or can involve secondary operations. Typically, once the parison is extruded between the mold halves, it is pinched off at the top and/or bottom and then blown into a part that takes the shape of the mold cavity. The pinch-off generates excess plastic material in the form of top and/or bottom flash.
Flash generation imposes limits on blow molding efficiency. Flash removal typically calls for a post-molding trim step, which requires special equipment and poses a risk of damaging good parts. Additionally, flash has potential for significantly extending the molding cycle, primarily by increasing the time needed to cool the thick flash areas. Another common undesirable feature of the flash generation is that, during the process of flash removal, an inner wall of the article dips inward at the parting line. This dip is created when the pinch-off feature on the mold closes onto the plastic parison. This common characteristic results in leaks whenever the inner surface of blow molded parts are mated or coupled for use as conduits for water, air and other fluids.
There are a number of prior art solutions to the problem of flash generation. One such solution is to keep the parison inside the entire perimeter of the blow molded part. However, this is technically impossible during the extrusion blow molding. Although the two-stage process for injection blow molding is often used to eliminate flash, it is impractical for many blow molded article shapes. The best expected outcome from extrusion blow molding is for the parison to enter the mold from one end (usually the top) and exit from the other (usually the bottom). This approach limits the indentation from the mold closing on the parison to top and bottom and provides side seams that do not indent since the flash is kept inside the part in those areas.
Another prior art solution is to compress the flash attempting to squeeze parison back into the seams at the parting line between the two mold halves. However, this method has a processing disadvantage in that the amount of compression will inevitably vary from part to part. As a result, some blow molded parts will leak from indentation and some will leak from protrusion. Either way, it is not consistent enough to prevent leaks near the parting line on the inner surface of the blow molded article.
Yet another prior art solution is to create a so-called “bottle neck” like feature using a blow pin or ram. This process requires the blow pin to be positioned inside the parison as it is being extruded so the location of the round inner wall is limited to top or bottom openings in the parison. However, this process requires an extra step during the blow molding process, thereby making the process more costly and less efficient.
What is further desired, therefore, is a system and method for creating a cut in a blow molded article, wherein the cut is made while the article is still inside the mold. What is also desired is a system and method for creating blow molded articles with shapes that require flash that avoids the indentation at the parting line between the molded halves. What is further desired is a system and method for creating blow molded articles wherein the shape of the opening and the inner wall of the article can be specifically varied and controlled. What is finally desired is a system and method for in-mold finishing of blow molded articles that is simpler and more efficient than the prior art systems.